The present invention relates to a tire with radial carcass reinforcement of low H/S form ratio, which tire is particularly intended to be fitted on a heavy road vehicle.
Such a so-called "heavy vehicle" tire comprises a radial carcass reinforcement, generally formed by a single ply of radial metal cables preferably made of steel, said ply being anchored in each bead to at least one bead wire, which itself is made of metal, having a composition and a shape matched to the inclination of the bead seat, in order to form an upturn. The skeleton of the tire is completed by a crown reinforcement which hoops the carcass reinforcement. Said crown reinforcement is generally composed, radially from the carcass reinforcement to the outside, of a ply or two half-plies called triangulation plies, two plies called working crown plies and one or two plies called protective plies. The triangulation halfply or plies, the object of which is to prevent the subjacent cables of the carcass reinforcement becoming compressed, are formed of inextensible metal cables, forming a large angle, possibly being between 50.degree. and 90.degree., with the circumferential direction. The working crown plies are formed of inextensible metal cables, crossed from one ply to the next, forming with the circumferential direction angles, which may be equal or not equal, which are generally small, of between 15.degree. and 35.degree.. As for the protective plies, they are formed of elastic cables which form with the circumferential direction angles o f a value similar to that of the angles of the cables of the working plies.
Such a crown reinforcement structure is very widely used in the "heavy vehicle" tires of H/S form ratio .gtoreq.0.8, and is completely satisfactory. On the other hand, for many reasons it is difficult to apply to tires of H/S form ratio &lt;0.60.
In order to solve one of the problems set, namely controlling the meridian profile of the tread, it has been proposed, for example in French Patent 2 566 334, to replace the triangulation ply (plies) with a ply of inextensible circumferential or quasi-circumferential cables, the axial ends of which are axially on the outside of the ends of the two plies of crossed cables, which complete the crown reinforcement of the tire described in the afore-mentioned patent.
For the same reasons as previously, French Patent 2 419 185 recommends the use of two hooping reinforcements located on either side of the equatorial plane, each reinforcement being composed of two plies of low width and of inextensible metal cables crossed from one ply to the other, forming a preferably small angle of 5.degree. to 0.degree. with the circumferential direction.
Although such crown reinforcement structures have the advantage of greatly reducing the risk of separation between the ends of the working plies, at the same time as they make it possible to avoid the formation upon inflation of the concave shape of the tread, on the other hand they are not sufficiently effective for overcoming a specific problem of the dimensions of the tire in question, namely the risks of the cables of the carcass ply becoming compressed, a ply of circumferential cables not being at all effective in solving such a problem.
It is possible to overcome the above disadvantages by various means, for example by redimensioning the cables of the carcass ply so as to make them more resistant to fatigue by compression. Another means consists in controlling the transverse contraction of the crown reinforcement. The current known means, generally consisting in increasing the metallic mass of the reinforcements, result in an increase in the cost of the tire.